Simple sample for measuring coin cell battery voltage with Zephyr and nRF52840

Question

My project uses Zephyr and the nRF52840 with a CR2450 coin cell battery. All is working well, and now I am looking to add in battery level. I've downloaded and read the following samples. 
 zephyr/samples/boards/nrf/battery zephyr/samples/drivers/adc 
 These samples seem to focus on particular hardware (like the Thingy) that use lithium batteries, and so they have a /vbatt voltage divider to bring the voltage down to a range the nRF52840 can read. There are some ifdef and devicetree mechanisms that imply they could work without a voltage divider, but I'm getting confused by the samples. 
 My board is powered directly by the CR2450, and so I believe I don't need a voltage divider. My board also has P0.02/AIN0 directly connected to the coin cell (3.3V nominal) in anticipation of using this pin for reading the voltage. 
 If I could read the voltage, I'll do the math to determine battery level. 
 My questions are as follows. 
 - From parts of the samples, I'm guessing that the nRF52840 has an internal way to measure VDD. (See "zephyr,input-positive = <NRF_SAADC_VDD>;" in the nRF52840DK devicetree overlay in the "add" sample.) Is this true? If so, how do I access it? 
 - If the above is not true, could my connection between P0.02/AIN0 and the coin cell be used to measure its voltage? 
 - Is there a sample, or some guidance within an existing sample, that you can give me to make this clearer? 
 Thanks, 
 Steve

abvio · Accepted Answer

Hi Hieu,

Thank you for your follow up. I'm happy to report I have it completely solved and am reading the internal VDD using the ADC.

It still wasn't clear why my code wasn't compiling, but after iterating on a lot of possibilities, I did change the sample.yaml in my project to match the sample.yaml in the ADC example, which mentioned "adc". After that and a few more iterations, the code started to compile. And after some further debugging, it started to work.

In case someone is looking for some simpler sample code, I reduced the devicetree to just one channel that looks like this:

/ {
	zephyr,user {
		io-channels = <&adc 0>;
	};
};

&adc {
	#address-cells = <1>;
	#size-cells = <0>;
	status="okay";

	channel@0 {
		reg = <0>;
		zephyr,gain = "ADC_GAIN_1_6";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,input-positive = <NRF_SAADC_VDD>;
		zephyr,resolution = <14>;
		zephyr,oversampling = <8>;
	};
};

The code I use to access it looks like this:

#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
	!DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
#error "No suitable devicetree overlay specified"
#endif

#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
	ADC_DT_SPEC_GET_BY_IDX(node_id, idx),

/* Data of ADC io-channels specified in devicetree. */
static const struct adc_dt_spec adc_channels[] = {
	DT_FOREACH_PROP_ELEM(DT_PATH(zephyr_user), io_channels,
			     DT_SPEC_AND_COMMA)
};

K_MSGQ_DEFINE(battery_msgq, sizeof(battery_event_t), 10, 4);

int32_t battery_level(void) {
	int err;
	int32_t val_mv;
	int16_t buf = 0;
	uint8_t level = 0; 

	struct adc_sequence sequence = {
		.buffer = &buf,
		/* buffer size in bytes, not number of samples */
		.buffer_size = sizeof(buf),
	};

	if (!device_is_ready(adc_channels[0].dev)) {
		printk("ADC controller device not ready\n");
		return level;
	}
	err = adc_channel_setup_dt(&adc_channels[0]);
	if (err < 0) {
		printk("Could not setup channel #%d (%d)\n", 0, err);
		return level;
	}
	(void)adc_sequence_init_dt(&adc_channels[0], &sequence);

	err = adc_read(adc_channels[0].dev, &sequence);
	if (err < 0) {
		printk("Could not read (%d)\n", err);
		return level; 
	}

	val_mv = buf;
	err = adc_raw_to_millivolts_dt(&adc_channels[0], &val_mv);
	if (err < 0) {
		printk(" (value in mV not available)\n");
		return level; 
	}

// Voltage to level curve for a coin cell 3V battery. 
#define BATTERY_FULL_MV 2950
#define BATTERY_HALF_MV 2900
#define BATTERY_QUARTER_MV 2800
#define BATTERY_DEAD_MV 2300

	if (val_mv >= BATTERY_FULL_MV) {
		level = 100; 
	} else if (val_mv <= BATTERY_DEAD_MV) {
		level = 0; 
	} else if (val_mv >= BATTERY_HALF_MV) {
		level = (val_mv - BATTERY_HALF_MV) * 50 / (BATTERY_FULL_MV - BATTERY_HALF_MV) + 50;
	} else if (val_mv >= BATTERY_QUARTER_MV) {
		level = (val_mv - BATTERY_QUARTER_MV) * 25 / (BATTERY_HALF_MV - BATTERY_QUARTER_MV) + 25;
	} else {
		level = (val_mv - BATTERY_DEAD_MV) * 25 / (BATTERY_QUARTER_MV - BATTERY_DEAD_MV) + 0;
	}

	return level; 
	// return val_mv / 100; 
}

Thanks again for your help,

Steve

Simple sample for measuring coin cell battery voltage with Zephyr and nRF52840

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